The present invention relates to a drive device for a light emitting diode (LED) or other light emitting element emitting light with a luminance in accordance with a flow of current and a display system having for example a non-light emitting transmission type display device using the same.
The mainstream of backlights of liquid crystal panels is the cold cathode fluorescent lamp (CCFL) type using a fluorescent lamp, but a mercury-less type has been demanded for protection of the environment. Due to this, in recent years, LEDs have been looked at as promising light sources to take the place of CCFLs. In particular, the method of individually using primary colors of red LEDs, green LED, and blue LEDs and optically combining them to obtain a white color enables easy balance of color, therefore use for the purpose of television is being actively studied.
An LED basically has the characteristic of changing in luminance in accordance with the current. Further, the forward direction voltage fluctuates due to individual differences and temperature. Accordingly, when using LEDs for the backlight of a liquid crystal panel (LCD etc.), a constant current characteristic is sought for the drive devices in order to ensure a constant, uniform luminance.
Simply speaking, as shown in FIG. 1, the method of limiting the current by connecting each LED 2 in series with a resistor 3 to an output of a constant voltage source 1 is known. In the system of FIG. 1, however, with a high luminance LED through which a large current is passed, the loss due to this resistor 3 becomes an issue.
As a method for dealing with this, as shown in FIG. 2, there is the method of using a constant current source 4 as the drive device. In this drive device, adjustment of the luminance of the LED 2 is possible by reducing the current value. The current value is generally changed by inserting a resistor 5 in series to the LED 2, detecting the current value by the potential difference across the ends, applying feedback control, and controlling the current value to a target value. However, the smaller the current, the smaller the potential difference, so the worse the luminance and the greater the susceptability to noise etc. Further, when the resistance value is made large enough that a sufficient voltage is obtained even with a small current, there is the disadvantage that the loss becomes large at the time of a large current.
Therefore, in order to stably adjust the luminance in a wide dynamic range, there is known a drive device employing the PWM control system of turning ON/OFF the current flowing in the LED at a constant timing and adjusting the luminance according to a ratio of that ON/OFF period. As one of methods accomplishing this method, as shown in FIG. 3, a method of inserting a switch element 6 in series to the LED 2 and turning it ON/OFF at a certain determined timing is employed (see for example Japanese Patent Publication (A) No. 2001-272938).
However, when using a constant current source as a drive device employing the PWM control system, it is attempted to create a constant flow of current at a constant current source irrespective of the fact that the current does not flow in the OFF period, therefore the control becomes abnormal and there is the apprehension that correct constant current control can no longer be carried out. As a result, there is the disadvantage that stable adjustment of the luminance adjustment is not possible.